The active movement of cations and anions in a large number of systems have been investigated quite extensively, mainly K ion, Na ions, Ca ions, and Cl minus transport. Movements of the NH4 ion have not received as much attention with the exception of the probable passive movements of NH4 ion in the kidney. The ammonium ion as such is important because of its relationship to body fluid acid-base balance, its alteration of macromolecule synthesis, its role in ammonia intoxication during hepatic failure and its competition with the movement of other cations. This proposed research will investigate the active movements of the NH4 ion using the isolated hindgut of the blowfly larva, Sarcophaga bullata, as a model system. The isolated hindgut has the advantage of being relatively simple in structure as compared with other systems involved in ammonium movements, and it functions in vitro, maintains a very large NH4 ion concentration gradient, and much of the preliminary characterization of the system has already been done. Attempts will be made to elucidate the mechanism of NH4 ion transport in cellular and transcellular electrical potential measurements, ion fluxes and interactions of NH4 ion with the ion fluxes and potentials, and the effects of metabolic inhibitors on these various paramaters. Information gained from studies on the mechanisms of NH4 ions in the isolated hindgut system should be directly applicable to the movement of NH4 ion movements in other biological systems including the mammalian gut and kidney.